1. Field of the Invention
The present invention relates to an improved process for continuous hydrogenation of adiponitrile (ADN) to hexamethylene diamine (HMD) and aminocapronitrile (ACN). More specifically but not by way of limitation, the present invention relates to a process for the continuous hydrogenation of ADN at low temperature and pressure in the presence of a sponge cobalt catalyst and in a reaction medium substantially free of caustic.
2. Description of Related Art
It is generally known in the art that the hydrogenation of nitriles can be achieved by contacting a hydrogenation catalyst with a solution containing a nitrile, hydrogen, and an alkali or base. The reaction product typically contains primary amine as the desired product and one or more secondary and tertiary amines as adventitious side products. The secondary amines are believed to be formed by the reaction between primary amines and imine intermediates, formed by the partial hydrogenation of the primary amines. Tertiary amines, on the other hand, are the products of the reaction between secondary amines and additional imine intermediates. Various alternative methods for accomplishing such hydrogenation have been proposed including batch-processing in a stirred autoclave reactor, or by continuous processing, using a fixed-bed or bubble-flow reactor.
For example, in U.S. Pat. Nos. 3,758,584 and 3,773,832 a fixed-bed reactor has been used for the hydrogenation of ADN. HMD was prepared by hydrogenating ADN in packed bed reactors in the presence of excess of hydrogen and anhydrous ammonia, at temperatures of 85 to 185.degree. C. and pressures of 4,000 to 6,000 psig. Cobalt or iron oxide was pelletized, calcined, and reduced in a mixture of hydrogen and ammonia at a temperature in the range of 300 to 600.degree. C. The major drawbacks of this process are that it requires high temperature and pressure, a large amount of ammonia, reduction of iron or cobalt oxide catalyst used in this process, and difficulties associated with heat removal. Moreover, the cost of the catalyst used in the process has been increasing over time.
U.S. Pat. Nos. 4,429,159 and 4,491,673 describe the use of a Raney.RTM. nickel catalyst in a bubble-column reactor. The described process uses lower pressure (200 to 500 psig) and temperature (less than 100.degree. C.). However, this process uses copious amounts of caustic to maintain the activity, selectivity, and the life of the catalyst. As a result, it requires expensive purification steps to produce high purity HMD.
A process for preparing HMD is disclosed in U.S. Pat. No. 5,105,015 where HMD is produced in a fixed bed reactor by hydrogenating ADN in the presence of granular Cr and Ni promoted Raney.RTM. Co catalyst. The reaction is carried out in the presence of at least 5 wt % ammonia (based on ADN) at a temperature and pressure in the range of 60 to 125.degree. C. and 50 to 5,000 psi, respectively. The disadvantage of such a process is the inherent necessity to recover, purify, compress, and recycle ammonia (used as a solvent).
The preparation of a branched aliphatic diamine from the corresponding dinitrile is described in U.S. Pat. No. 4,885,391. This patent discloses the hydrogenation of 2-methylglutaronitrile to 2-methyl, 1,5-pentamethylenediamine using chromium-promoted sponge cobalt catalyst in the presence of 0.5 to 4.0% by weight of water, at a temperature of about 80 to 150.degree. C., and at a pressure of about 400 to 2,500 psig. While it is not obvious whether a branched aliphatic dinitrile will behave in the same fashion as a straight chain aliphatic dinitrile, it is apparent that the process makes a number of by-products including some cyclic condensation products. This would lead to a lower yield of the desired product during reaction, as well as separation. In addition, there would be a significant amount of water present that would need to be continuously pumped into the reaction vessel, to sustain the activity of the catalyst.